Audaspace Game Property in Python script doesn't work as expected
Your mistake is in logic.device.play(sound.lowpass(own["Amount"])) you are missing the frequency value and the amount go from 0.0 to 1.0 (step=0.1)

From the Blender aud API:


  lowpass(frequency, Q=0.5)
  
  Creates a second order lowpass filter based on the transfer function H(s) = 1 / (s^2 + s/Q + 1)
  
  Parameters:
  
  frequency (float) – The cut off trequency of the lowpass.
  
  Q (float) – Q factor of the lowpass.
  
  Returns: The created Factory object.
  
  Return type: Factory


Test this (i haven`t test it): 

import bge
from bge import logic
import aud

cont = logic.getCurrentController()
own = cont.owner

logic.device = aud.device()
soundpath = logic.expandPath('//Sounds/Engine.wav')
sound = aud.Factory.file(soundpath)
fequency = 100.0 #Hz i assume
amnt = own["Amount"] * 0.1
sound = sound.lowpass(frequency,amnt)
handle = logic.device.play(sound)
handle.volume = 1